This invention relates to electrostatic discharge protection circuits (ESD protection circuits) for integrated circuit chips.
Basically, in an integrated circuit chip, large metal pads are provided on which discrete wires are bonded to thereby provide a means whereby input signals can be sent to and output signals received from the chip. Those signals, under normal operating conditions, are restricted to lie within a certain voltage range. Typically, that voltage range is .+-.5 volts, or even smaller. However, due to electrostatic charge, the voltage on the pads can, for a short time period (e.g., a few nanoseconds), become 1,000 volts or higher.
Such electrostatic charge first accumulates on one's body. A simple circuit, which approximates the equivalent circuit for the human body, is a 100 picofarad capacitor in series with a 1,500 ohm resistor. By accumulating a charge of just 1.times.10.sup.-7 Coulumb's of charge on this capacitor, the voltage across it becomes 1,000 volts. This amount of charge, either positive or negative, can readily be accumulated on the human body. Thereafter the charge will be transferred to a pad when the person contacts either the pad, or a wire to which the pad is connected, with a probe.
When this charge is transferred to a contact pad on an integrated circuit, a large current can flow onto the chip and there burn out any transistors which are connected to the pads. So to address this problem, various static discharge protection circuits have been proposed in the prior art. See, for example, U.S. Pat. Nos. 4,481,421 and 4,605,980, and 4,686,602. However, the protection circuits of these patents, and others, have serious deficiencies.
One problem is that the previously disclosed protection circuits all occupy at least some chip space which detracts from the space which is available for the remaining circuitry on the chip. Consequently, there is a tendency to reduce the size of the components which make up the input protection circuitry. But that in turn decreases the current-carrying capacity and increases the series resistance of the input protection components. This then presents a real dilemma,--if the components are made too small, they will burn out; and if they are made too large, they will occupy too much chip space. Further, the prior art protection circuitry, as will be shown in detail later, is not located where the electrostatic charge tends to accumulate, and so its effectiveness in removing that charge is reduced.
Accordingly, a primary object of the invention is to provide an improved static discharge protection circuit for integrated circuits in which all of the above problems are overcome.
In accordance with the invention, an integrated circuit chip having improved static discharge protection comprises a semiconductor substrate with a major surface, a plurality of transistors that are integrated into the surface, patterned conductors that interconnect the transistors and route input signals to the transistors, with the patterned conductors including metal pads for receiving the input signals from an external source; wherein the improvement comprises respective diodes which are integrated into the surface directly beneath the metal pads, and which connect the pads to the substrate and conduct electrostatic charge therebetween. With this structure, no additional chip space is required over that which is used by the transistors which are being protected since the diodes are hidden in the normally unused chip space beneath the pads. Also with this structure, the diodes can be large since the metal pads are inherently large enough to receive a bonding wire; and thus the diodes have a large current-carrying capacity and a small series resistance. Preferably, the metal pads have sharp corners of 90.degree. or less which tend to accumulate any electrostatic charge, and the diodes are disposed beneath the metal pads at these corners. Since the diodes are located where the charge tends to accumulate, they are more effective in dissipating that charge than if they were located elsewhere.